It so happens he wrote that sentence in closing a column on President Obama's elaborate story about the Navy SEALs' May 2, 2011, assassination of Osama bin Laden.
Cockburn wrote, 

There was scarcely a sentence in the President's Sunday night address, or in the subsequent briefing by John Brennan, his chief counter-terrorism coordinator, that has not been subsequently retracted by CIA director Leon Panetta or the White House press spokesman, Jay Carney, or by various documentary records.

Cockburn's column was based on reporting that undermined key details of the official narrative. For example:

The official "back story" released Sunday night by Obama is that US intelligence learned of the Abbottabad compound only last August and spent the following months watching the place, following Osama's trusted couriers and concluding that it was highly likely, though not certain, that Osama was there.

This is bunk. The three-storey house has been a well-known feature of Abbottabad. Shaukat Qadir, a well-connected Pakistan Army officer, reported to CounterPunch from Pakistan: "For the record, this house has been under ISI [Pakistani intelligence] surveillance while it was under construction."